THE ELASMOBRANCII FISHES 



127 



DIGESTIVE TRACT OF ELASMOBRANCHS IN GENERAL 



The digestive tract const itutes a tube in which food is digested and through 

 the walls of which it is absorbed into the circulatory system. In the adult, as 

 we have seen in Heptanchus, the tract is folded upon itself so that when seen in 

 ventral view it takes the form of an |. A median line parallel to the body axis 

 may bisect the oesophagus and cloaca leaving the stomach to the left and the 

 spiral intestine to the right. 



Mesenteries 



The mesenteries of Heptanchus are generalized when the Elasmobranchs as a 

 group are considered, but they are more specialized than are those described 



Fig. 125. Development of teeth in lower jaw of Spinux niger. (From Laaser.) 



d.r., dental ridge; e., enamel; e.o., enamel organ; md., mandibular cartilage; n.f., outer 

 furrow ; p., tooth papilla. 



by Howes (1890) for Hypnos suhnigrum., the Australian torpedo. In Hypnos, 

 which has the most generalized type of any adult Elasmobranch with which I 

 am acquainted, the mesentery extends as an almost unbroken sheet along the 

 entire digestive tract. Only in the region of the spleen and of the rectal gland 

 are there any indications of a break in the folds. In the adult of most other 

 Elasmobranchs, however, there is a more pronounced tendency than in Hep- 

 tanchus toward a loss of parts of the folds. In Acanthias, for example, rela- 

 tively small parts of the folds which we have described as right and left remain. 

 In its development the digestive tract of the Elasmobranch fishes is a more 

 or less simple tube. It consists of a median segment which is put into com- 

 munication with the outside : ( 1 ) l)y an anterior invagination which finally, 

 as the stomodeum, ])reaks through to join the middle segment, and (2) by a 

 posterior pit which also reaches the middle segment as the proctodeum. There 

 is thus formed a tube including three parts: (1) the anterior stomodeum, 

 lined with the outside ectoderm, which in the adult becomes the buccal and 



